The present invention relates to a semiconductor laser device that is used for an optical recording information device such as CD-ROM, CD-R/RW, MO and DVD, and a method for producing the same.
As a semiconductor laser device in which a semiconductor laser element is mounted on a stem, there has hitherto been one shown in FIGS. 4A and 4B. FIG. 4A is a perspective view showing the exterior of the device, and FIG. 4B is a longitudinal cross-sectional view thereof. A semiconductor laser element 1 and a monitoring photodiode (not shown) are die bonded to a submount 4 of a stem 3 and they are connected to lead pins 6 using Au wires 5. Further, a cap 8 is welded to the stem 3 by electric welding, and for the purpose of protecting the semiconductor element from dust from outside, external stress and/or optical interference, the cap 8 is provided with a window opening 7 letting laser beams pass therethrough (e.g., see JP-2000-252575A).
Welding between the stem 3 and the cap 8 is carried out as follows. That is, a flange 9 formed by bending an edge portion of the cap 8 outwardly is placed on an upper surface 10 of the stem 3. Then, as shown in FIG. 5, while applying a pressure to an upper side of the flange 9 and a lower surface of the stem 3 using an upper electrode and a lower electrode, a large current is instantaneously passed via wiring 14. Thereby, a junction portion between the flange 9 and the stem 3 is heated to a high temperature and part of the junction portion is melted to be welded (e.g., see JP-2000-252575A). Particularly, in the case where the junction portion requires hermeticity, the cap 8 is circumferentially provided with a separate flange for welding. Further, in order to carry out welding stably, it is required that the width of the flange 9 be increased.
As shown in FIG. 6, when a circular or oval semiconductor laser device 15 is incorporated in an optical pickup, in order to reduce the height of a space underneath a disc 16 as much as possible, an optical axis 18 is bent at a right angle by using a mirror 17 or a prism, and a laser beam is focused by an objective lens 19 on the disc 16 positioned above.
The semiconductor laser device 15 is constructed in a manner so as to be able to rotate about the optical axis 18 for the purpose of adjusting variations in various characteristics including polarization characteristics due to the fact that emitted light from the semiconductor laser element is spread not in a circular shape, but in an ellipse shape having different spread angles between vertical and horizontal directions. In order to rotate or position the semiconductor laser device 15, the stem 3 is provided with V- or U-shaped notches.
If the position of a light-emitting point is changed when rotating the semiconductor laser device 15, various problems occur. Therefore, in order to prevent the light-emitting point from being changed, the stem 3 is formed in a circular shape or an ellipse shape in which part of the circular shape is left, so that the light-emitting point is positioned at the center of the circular portion.
However, the conventional semiconductor laser device has the following problem. That is, if the light-emitting point is positioned at the center of the circular portion because of the necessity of rotating the semiconductor laser device 15, a distance H between the optical axis 18 and a lower edge portion of the stem 3 is indispensably required, which is a cause of hampering a reduction in the thickness of the optical pickup.
For that reason, in order to reduce the above distance H, stems of which the outer size (diameter φ) is reduced to 9 mm, 5.6 mm, 3.5 mm, and 3.3 mm are suggested every year. D- or ellipse-shaped stems formed by cutting off part of their circular portions have also been devised. However, when the semiconductor laser device is rotated for optical adjustment, the cut-off straight-line portions are not necessarily positioned at the bottom of the optical pickup. Thus, miniaturization corresponding to the cut-off portions is not secured.
In the case where the external shape of the stem 3 is circular, the miniaturization also reduces the outer diameter of the cap 8. However, errors in sizes of members, errors in sealing a cap (eccentricity degrees), sagging at edge portions of the stem 3 and so on are not reduced. Thus, a region of the upper surface 10 of the stem 3 that is exposed from the flange 9 of the cap 8 and is used as a reference plane 21 is further reduced. As shown in FIG. 7, a practically effective width, Weff, of the reference plane 21 is much shorter than a value, Wcal, obtained by simply subtracting the radius of the cap 8 (including the flange 9) from the radius of the stem 3 (i.e., a calculated width of the reference plane). This becomes more conspicuous as the outer diameter of the stem 3 decreases.
FIG. 8 shows the relationship between the deviation angle θ of the optical axis 18 from the normal line to a reference plane 22 of the optical pickup, which angle is attributable to unevenness (flatness), d, of the reference plane 21 shown in FIG. 9, and the diameter of the stem 3. In this case also, the influence of a raised portion of the reference plane 21 on the deviation angle θ becomes larger as the diameter of the stem 3 decreases. Therefore, as the diameter of the stem 3 becomes smaller, the deviation angle θ becomes larger. In this manner, as the region serving as the reference plane 21 becomes smaller, even slight unevenness, d, would make the deviation angle θ of the optical axis 18 larger. In general, it is required that the deviation angle θ of the optical axis 18 in the semiconductor laser device 15 be in the range of from 1° to 1.5°. Previously, a big factor to the deviation angle θ was the die bonding accuracy of the semiconductor laser element 15. Recently, however, as the diameter of the stem decreases, the area of the reference plane 21 becomes a problem.
In welding between the cap 8 and the stem 3, if the flange 9 to be welded is formed uniformly in a circular shape, an applied current uniformly flows, so that welding is uniformly carried out. However, if the flange 9 is not uniform, uniform welding may not be carried out due to differences in pressure-applying conditions at welding portions and contact conditions such as surface conditions.
On the other hand, in a non-hermetic type semiconductor laser device in which the window 7 of the cap 8 is not closed by a permeable material such as glass, when incorporating it into an optical pickup, there is a case in which a hermitically semi-hermetic state occurs due to adhesion of an adhesive resin to the window, which resin is used when fitting an optical component or fixing the semiconductor laser device itself. When such a semi-hermetic state occurs, air with high humidity that has entered the cap due to fluctuations in temperature may condense, instead of going out of the cap, which results in deterioration of the semiconductor laser element, or which adversely affects the optical characteristics remarkably.